It was a challenge that imagined the veil as a gateway into all other possible universes -- so when Sirius fell through, he ended up in a place where, say, he had died instead of James and Lily, or in a place where Remus was a Muggle, or whatever. It was a fun way to showcase a bunch of AUs, at the end of which he would return to the veil to try to get back to his Remus.

It has disappeared from the internet, but fortunately the Wayback Machine has it: http://web.archive.org/web/20071010061909/http://broyc.laudatio.com/veil/archive.htm

The individual stories vary significantly in quality -- my favorite is the first one. But every time I reread it I get angsty over the fact that no one ever wrote Sirius coming home!